The present invention is directed to an expansion anchor assembly to be secured in a borehole. The assembly is made up of an axially extending expansion sleeve and an axially extending anchor rod. One end of the expansion sleeve is formed by a plurality of shell-shaped expansion parts separated by axially extending slots. The expansion parts can be deflected radially outwardly by drawing an expansion cone on one end of the anchor rod into the expansion sleeve so that the expansion parts are displaced around bending locations.
An expansion anchor assembly to be fastened in a borehole with a conically widened undercut is disclosed in U.S. Pat. No. 4,519,735. The assembly includes an expansion sleeve with shell-shaped expansion sections and an anchor rod with an expansion cone. The expansion sections are deflected radially by drawing the expansion cone into the expansion sleeve and displacing the expansion section into a positive-locking engagement. In such an expansion anchor assembly, the expansion sections are displaced around a weakened section connecting the sections with the adjacent portion of the expansion sleeve.
The inner and outer surfaces of the expansion anchor assembly are cylindrically shaped, that is, they have a constant radius of curvature. The inside radius of curvature of the expansion section corresponds to the radius of curvature of the expansion sleeve opening adjacent to the expansion sections. Similarly, the outside radius of curvature corresponds to the radius of curvature of the cylindrical outer surface of the expansion sleeve.
When the expansion cone is drawn into the sleeve, it is centrally located between the expansion sections with the radius of curvature increasing toward the free end, whereby the inner surface of the expansion sections is radially supported on the expansion cone only along axially extending edges spaced apart in the circumferential direction due to the constant radius of curvature. As a result, the surface of the expansion sections has a concave shape with respect in the expansion cone. In a like manner, the expansion sections only contact the conical surface of the undercut with line contact, because the expansion sections have a constant radius of curvature on the outside, while the radius of curvature of the widened undercut increases toward the base of the borehole. Consequently, the expansion sections contact the undercut conical surface along an axially extending line.
Very high specific compressive stresses develop between the expansion sections and the expansion cone and the wall of the widened undercut, during the loading of the expansion anchor assembly, since the inner and outer contact of the expansion sections is only a line contact. In particular, such contact can lead to overstressing the line contact locations in the undercut supporting the expansion sections, and, accordingly, to impairment of the anchorage. It is not possible to obtain a surface area contact between the expansion sections and the undercut surface after the radial deflection of the sections, since excessive forces would be necessary for deformation of the rigid expansion sections for adaptation to the conical surfaces of the expansion cone and of the undercut.